The IgG2a monoclonal antibody SWA11 has been evaluated as a radioimmunotherapeutic agent for use in the treatment of small cell cancer of the lung. This antibody was initially selected for in vivo localization studies in a nude mouse model system because of its high affinity for the SW2 small cell cancer cell line in vitro. Following i.v. injection of 125I labeled antibody into nude mice bearing SW2 xenografts good selective accumulation was observed with 10.5% of injected material/g of tumor. The level remained constant from day 2 to day 4 following injection. At day 4 the tumor:blood ratio was 7:1 and tumor:liver, tumor:kidney, and tumor:lung ratios were 17:1, 24:1, and 12:1, respectively. Initial radioimmunotherapeutic studies performed on established small cell cancer of the lung xenografts have shown reduction in tumor burden following a single injection of 300 microCi of 131I labeled SWA11 with no evidence of regrowth up to day 34 postinjection. Histological evaluation of treated tumors revealed large areas of necrosis and extensive fibrosis. A few residual cells of tumor origin could be observed and these displayed atypical morphology. The clonogenic potential of such cells remains to be determined by long term observation.